Manufacture and refinement of chemical pulps and regeneration of the spent chemical liquors



1937. H. K. MOORE March 2, 2,072,177 MANUFACTURE AND REF/INEMENT OF CHEMICAL PULPS AND REGENERATION 0F THE sPEN'r CHEMICAL LIQUoRs 'Filed Oct. 24, 1935 3 Sheets-Sheet 1 C da??? m' March 2, 1937. H. K. MOORE n. 2 D N A S D.. L U P. L A C T. M E. H C F O T N E M E N I F E P... D N A E R U T C A F U N A M 3 Shets-Sheet 2 s ,R o U Q I L L A C I M E H C T N E D.. s E H T F 0 N O I T A R E N E G. E R

Filed Oct. 24, Y 17935 k if' March 2, 1937. v H. K. MOORE 2,072,177

` MANUFACTURE AND REFINEMENT OF CHEMICAL PULPS AND REGENERATION OF THE SPEN'I CHEMICAL LIQUORS Filed oct. 24, 1935 s sheets-sheet s Patented Mar. 2, 1937 UNITED sTATEs PATENT oFFlAcE MANUFACTURE AND REFINEMENT OF CHEIVIICAL PULPS AND REGENERATION F THE SPENT CHEMICAL LIQUORS Hugh K. Moore, Berlin, N. H., assignor to Brown Company, Maine Application October 24,

18 Claims.

applicable to various kinds of spent liquors of the foregoing class which are subjected to a regenerating process linvolving the smelting of the inorganic compounds of the spent liquors and the causticization of the smelted inorganic compounds, nevertheless, they have particular utility and advantage in the regeneration of spent socalled kraft cooking liquors and spent liquors re- 20 sulting from the step of refining preliberated cellulose pulps of the nature of kraft, sulphite, or the like, with refining liquors of a composition similar to that of the usual kraft cooking liquors, according to which causticization is performed on smelted compounds containing sodium carbonate andl sodium sulphate as lmain constituents.

In the production of kraft pulp, it is the practice to cook the raw Wood chips in an alkaline liquor whose active or fiber-liberating constituents are essentially caustic soda and sodium sulphide. Strictly speaking, the sodium sulphide does not act as such in the cooking liquor but hydrolyzes in water to furnish caustic soda and sodium hydrosulphide as the active agents ofv the cooking liquor, so that the cooking liquor is really richer in caustic soda than the formula showing the presence of sodium sulphide therein would indicate. The sodium hydrosulphide is not active in the same sense as is the caustic soda, notwithstanding generally held opinion to the contrary. The fact is that the sodium hydrosulphide does not split up the lignocelluloses into cellulose fiber and lignin to any appreciable extent, but it does influence the way in which the lignocelluloses are split up by the caustic soda in that it is a powerful reducing agent and inhibits the formation of oxycelluloses or other oxycellulose compounds from the cellulose ber as it is being liberated. The sodium hydrosulphide is'hence an active agent in the cooking liquor, but its function and activity is quite different from that of the caustic soda. Nevertheless, for the purpose of convenience, I shall hereinafter speak about the active -constituents of a Berlin, N. H., a corporation of 1933, serial No. 694,996

(c1. ca -46) kraft cooking liquor as consisting of caustic soda and sodium sulphide or sodium hydrosulphide. After a batch of raw wood chips have been reduced to pulp in a kraft cooking liquor, it is the practice to blow the digester in which the cooking `was effected Vand then to separate or Wash the spent cooking liquor from the raw kraft pulp in suitable. washing equipment. The separated spent cooking liquor is partly returned to the digesters and partly regenerated in the form of a solution of a mixture of fresh caustic soda and sodium sulphide which is admixed with the spent cooking liquor in the production of the kraft pulp.

Inv regenerating the spent cooking liquor, various processes have been followed, all of which are designed, however, to yield a solution containing a mixture of caustic soda and sodium sulphide. One well known regenerating process, which is disclosed in my Patent No. 1,137,780, dated May 4, 1915, involves concentrating the spent or black liquor to a solids content, including both organic and inorganic solids, of about 50%, at which content the liquor is heated in a confined system to a high internal pressure and is then exploded as a spraydirectly into a furnace for the purpose of effecting a combustion of the organic content of the liquor and a smelting of its inorganic content into a molten mass of inorganic compounds containing essentially sodium carbonate and sodium sulphide as the furnace effluent. Inasmuch as losses of sodium and sulphur constituents in the cycle must be made up, this is done by adding sodium sulphate (salt cake) to the vconcentrated black liquor before heating it under pressure. The molten or smelted inorganic compounds coming from the furnace are discharged into a body of water to form a hot solution which contains sodium carbonate, 'sodium sulphide, residual sodium sulphate, and smaller amounts of sodium thiosulphate, sodium sulphite, and sodium silicate. The hot solution thus formed is delivered to a causticizing tank and lime is added thereto so as to convert the sodium carbonate into caustic soda. The resulting causticized solution is then allowed to stand to ensure a settling of the precipitated calcium carbonate and other solids, is decanted, and

then delivered to a storage tank from which.

liquor is withdrawn as needed for the digesters. To the storage tank liquor is added the first washings of the precipitated calcium carbonate, whereas the last Washings serve for dissolving the smelted compounds, thereby ensuring as complete a recovery of sodium constituent as possible.

' carbonate and sodium sulphate.

`and ionization of the lime.

As a result of the foregoing recovery or regenerating steps orany other recovery procedures with which I am familiar, the cooking liquor inevitably contains a substantial proportion of inactiveV chemicals, largely in the form of'sodium Thus, a typical causticized solution produced according to the regenerating process of my patent may contain 28% or even very much more inactive chemicals in the form of sodium sulphate and sodium carbonate, based on the total solids or chemicals present in the liquor. These inactive chemicals are of no benefit whatever in the cooking liquor, but as a matter of fact detract from the pulping eiectiveness of the liquor and impair the quality of the resulting pulp. The presence of sodium sulphate in the spent cooking liquor also tends to cause trouble in the pipe lines, pump, and evaporating system through which the liquor must be passed in order to arrive at the concentration necessary for proper furnacing, as well as in the furnace itself, as will hereinafter appear in greater detail.

It is well Vfor the purposes of the present invention to scrutinize carefully the conditions that prevail during a causticizing operation, and that give rise to a causticized solution undesirably loaded, as hereinbefore described, with inactive chemicals. It is well known that the causticizing reaction is one of the mass action type, depending as it does for its completion upon the solubility of the reaction products and the solubility of the reactants. causticization may be Written as follows:-

The rst reaction does not go to completion in mill practice for the reason that lime, a reactant, is comparatively insoluble, being soluble to the extent of only 0.08 part in 100 parts of pure water at 100 C., whereas caustic soda, a reaction product, is infinitely soluble in water. The solubility of lime in water decreases progressively as caustic soda is progressively added thereto, inasmuch as the hydroxyl ions furnished to the water by the caustic soda represses the solution Accordingly, it will be clear that as the solution containing sodium carbonate is progressively being causticized, the

lsolubility of the lime effecting the causticization is progressively diminished until an equilibrium is reached when no further causticization of sodium carbonate will be effected even though sodium carbonate remains present in substantial amount in the solution and there isa large excess of lime in the sphere of reaction. In the case of sodium sulphate, there is some reaction to form calcium sulphate, but inasmuch as the calcium sulphate is more soluble than the lime, no precipitation of calcium sulphate can occur, though a small quantity of the more insoluble basic sulphate may be thrown out. It is for the foregoing reasons that in actual practice only about of the lime calculated to be theoretically necessary for the causticization of the sodium carbonate and the sodium sulphate present in the solution of smelted compounds is used.

4Such an amount of lime ensures all the precipiconditions of concentration of the causticized 4 solution, the caustic soda may throw out of solution some sodium carbonate and some sodium The reactions taking place during hydrosulphide, but this is not a serious matter as the crystals of these compounds remaining in the causticizing tank are dissolved in the water used for Washing the calcium carbonate sludge and are thus recovered.

In accordance with one phase of the present invention, causticization of the solution is performed as heretofore with lime, whereupon the work of causticization is completed through the use of barium sulphide and/or barium hydroxide as the causticizing reagents. Such practice is illustrated as a flow sheet in Figure l of the accompanying drawings. In this connection, I wish to point out that both barium sulphide and barium hydroxide are quite soluble in water, far more so than lime, .and that barium sulphate is far less soluble than calcium sulphate. ly stated, the causticized solution as it is delivered from the causticizing tanks contains caustic soda, sodium sulphide, and residual sodium sulphate and sodium carbonate. To this solution is added the barium constituent in the form of barium sulphide and/or barium hydroxide theoretically necessary to convert substantially all of the residual sodium sulphate and sodium carbonate into sodium sulphide and/or caustic soda, although it is preferable to use barium constituent slightly short of the theoretically calculated amount in order to avoid the presence of barium constituent in the cooking liquor and its oxidation into insoluble barium sulphate which would contaminate the pulp. The barium sulphate and/or barium carbonate precipitate thus produced may be separated from the completely causticized liquor and subjected to a reducing action in the presence of iinely divided carbon or carbonaceous material in a kiln to reproduce the barium sulphide and/or barium oxide used in the second stage of causticization. The reactions which take place in the kiln are as follows:

When the precipitate is essentially barium carbonate or a mixture of barium sulphate and barium carbonate, it may be desirable to carry out the reducing action in the kiln inthe presence of superheated steam so as to eiiect the following reaction:

In those instances when the kiln is heated by producer gas, the foregoing reaction will take place anyway because the hydrogen content of the producer gas burns to form superheated steam as one of the products of combustion.

In eifecting the last stage of causticization with barium sulphide, it should be noted that the products of such causticization consist of both caustic soda and sodium hydrosulphide, since barium sulphide hydrolyzes in water as follows:

'Strictly speaking, neither barium hydrosulphide nor barium hydroxide are formed as compounds which can be separated from each other, both coexisting as barium hydroxy hydrosulphide having the formula BaSHOH, so that when I speak hereinafter, or in the appended claims, about barium hydrosulphide, I mean thereby to include barium hydroxy hydrosulphide. This latter compound does the causticizing work on the residual sodium sulphate and sodium carbonate present The foregoing reactions tend to go to substantial completion by reason of the high solubility of the barium hydroxy hydrosulphide and the insoluble nature of the barium carbonate and the barium sulphate. In other words, the barium hydroxy hydrosulphide can complete the work of causticization initiated by the lime because it is far more soluble than the lime which, as previously explained, becomes more insoluble as the causticity of the solution increases until it is so insoluble that it can no longer effect causticization, at which time there is still a substantial amount of sodium sulphate and sodium carbonate left in the solution as well as residual undissolved lime. The question might be asked as to Why calcium hydrosulphide, which is extremely soluble in Water, cannot effect the same results as does the barium hydroxy hydrosulphide. The trouble with calcium hydrosulphide is, however, that upon being put into a solution ofcaustic soda, calcium hydroxide is immediately precipitated so that no better results are had than when lime is-used to begin with. The causticizing action effected by barium hydroxide is, as in thecase of barium hydroxy hydrosulphide, based upon the high solubility of this hydroxide and the insolubility of barium sulphate and bariumicarbonate. These reactions effected with barium hydroxide are well known lto the chemist and need not be Written herein.

There aremany advantages inhering in a process practiced along the foregoinglineswhereinlime is used to do all the causticizing Work that it can leo and barium sulphide and/or barium hydroxide are then used to complete the Work of causticization. Aside from thefact that the resulting causticized liquor is substantially free from inert chemicals like sodium sulphate and sodium carbonate that play no role whatever vin the pulping of wood or other raw cellulosic material,I it is possibleto control with accuracy the composition of the causticized liquor and thereby to reproduce a given quality of pulp. Thus, if the sulphidity of the causticized solution is too great, this can be adjusted by adding sodium carbonate to the solution being causticized and/or by operating the furnace in which the smelting is. effected under conditions to produce a greater amount of sodium carbonate. Indeed, the furnace may advantageously be run under oxidizing conditions substantially throughout torproduce essentially only sodium carbonate in the smelt,y inasmuch as it is possible through the use of barium sulphidev in the last stages of causticization to form the desired amount 'of sodium sulphide in the causticized solution. -This latter practice of operating the burning and smelting furnace without laying emphasis on the amount of sodium sul- 55 phide produced therein is of advantage in realize ing maximum fuel value from the organic content being-burned in a furnace of the type lde scribed in my aforementioned patent which is equipped with a boiler heated by the hot gaseous 70 products 'of combustion produced in the furnace.

Again, by running the furnace under oxidizing conditions, .the reaction may more nearly approach an exothermic one .throughout the furnace so4 that higher temperatures may be reached. in

75 thev furnace and accordingly ,'fgreater heating eiciency realized with a given fuel value in the liquor. The higher temperatures realized in the furnace also means that the furnace has ,greater productive capacity. Moreover, the more nearly Vone approaches an oxidizing atmosphere throughout the furnace, the less isy the wear and tear on the furnace walls.` Itis Well known that fused sodium sulphide formed by maintaining a reducing atmosphere in the furnace is destructive of practically all the materials of construction that can be used in making the furnace walls and that the best that can be 'done in practice is to select wall materials known to be most resistant to the chemical action of the fused sodium sulphide. Aside from the chemical-corroding action of the fused sodium sulphide, its physical action is also highly destructive in that it tends to enter into the pores and interstices of the lining and to oxidize therein into sodium sulphate, which oxidation is accompanied by expansion and tendency to erupt the' lining and slough off particles of lining'material at its internal face. The sodium sulphide also tends to sublime and deposit on the boiler tubes and in the flue, in which localities it undergoes oxidation to form sodium sulphate whichpiles up so as to necessitate its. frequent removal. The foregoing difficulties are minimized whenthe furnace'is run with an oxidizing atmosphere substantially therethroughout, al-

though some formation of sodium sulphide will f inevitably ensue when sodium sulphate (salt cake) is added to the concentrated spent liquor as it is being delivered to the furnace.

The troubles encountered in the usual process are not confined to the furnacing operation, but extend, as previously indicated, to the cooking or pulping operation, as the cooking liquor contains substantial amounts of sodium sulphate and sodium carbonate which, onaccoun't of their inactivity, tend to interfere with the activity of the caustic soda and the sodium hydrosulphide. These inactive chemicals also cause serious troublein other places in the system, as in the pipe lines, pumps, evaporators, explosion tanks, etc., for when the spent cooking liquor containing these chemicals reaches the last stages of concentration, these chemicals crystallize out and severely erode the metal parts with which they contact, particularly when the concentrated liquor containing the crystals is impinged forcibly as a spray against themetal parts, for instance, the metal parts of the last-stage evaporators, or is rapidly circulated, as it must be, past pipe and pump surfaces.v The fact is that this problem of erosion gives rise to the need of using special hard steels as the material of construction of the pipe lines, pumps, gill flanges, evaporator heads, explosion tanks, and the various other parts of the system. The seriousness of the problem is increased when the ratio of inorganic solids to organic solids present in the spent liquor is increased as a result of handling a spent liquor `vwhich consists of a mixture of spent kraft cooking liquor and spent pulp-refining liquor, as will presently be described.

According to another phase of the present in vention, I dispense with the addition of sodium sulphate to the concentrated spent liquor sent to the furnace to make up the losses of sodium, and sulphur constituents in the cycle. By so doing, it is possible to operate the furnace in such a. way as to realize an exothermic reaction there# throughout'and the production of a smelt subphate to the concentrated spent liquor, I convert such sodium sulphate into sodium sulphide by effecting a metathetical reaction thereon in solution with barium sulphide. The barium sulphide used for effecting this metathetical reaction on the make-11p sodium sulphate may be prepared, as hereinbefore described, by the reduction of barium sulphate in a kiln. The reaction products of the kiln contain not only barium sulphide, but also substantial amounts of residual barium sulphate and carbon. Nevertheless, the reaction products as a whole may be ground to a pulverulent condition, preferably in the presence of water to form a thick slurry, and the ground mass added to a solution of the make-up sodium sulphate to convert it into sodium sulphide. 'Ihe sludge thereby `formed containing barium sulphate and carbon is mixed with additional carbon and is returned to the kiln to reproduce barium sulphide. Should it be desired to produce substantially pure barium sulphate (blanc fix) as a by-product, the kiln-reaction products are leached in water, the leach solution separated from the residual solids, and the solution then treated with elemental, nely divided copper so as to effect a reduction of the barium polysulphides present in the leach solution to the monosulphide stage, as disclosed in my application Serial No. 670,882, led May 13, 1933. The desulphurization of the barium polysulphides present in the barium sulphide solution tothe monosulphide stage is attended by a change of color of the solution from a canary or orange yellow to a water-white; and, when this water-white solution is used to eiect'the metathetical reaction on the solution of make-up sodium sulphate, the resulting barium sulphate precipitate is of such -high whiteness and purity that it can be dehydrated and sold as blanc fix. Such new barium sulphate and new nely divided carbon as is needed in the process may be added to the sludge separated from the leach water, and the mixture introduced into the kiln for reduction.

The sodium sulphide solution separated from the sludge may be added to the lime-causticized solution 'of the-smelted compounds to produce a cooking liquor of the desired composition. In this connection, I wish to observe that when the make-up sodium sulphate 'is converted into sodium sulphide, as hereinbefore described, rather than being added to the -concentrated spent liquor, it is possible to perform the causticization of the solution of smelted compounds with lime at such dilution as to reduce materially the amount of inactive chemicals, i. e., the sodium sulphate and sodium carbonate, in the lime-causticized solution. This reduces the amount of barium sulphide and/or barium hydroxide necessary to effect the second stage of causticization, which is performed as hereinbefore described. Despite the fact that causticization is\ hus carried out so than the ordinary fresh cooking liquor. For instance, in a typical'kraft cooking liquor such as has heretofore beenused, pounds of the liquor may contain 8 pounds of active chemical, assuming that the sodium sulphide as well as the caustic soda is active, and about 3.2 pounds or considerably more of sodium sulphate, sodium pounds of cooking liquor and at the same time to realize a pulp of qualities superior to those of the usual kraft pulp. These improved qualities in the pulp are attributable to the fact that with the ordinary kraft cooking liquor, when the raw wood chips become saturated with cooking liquor, their pores and interstices are charged with inactive chemicals as well as with active ones so that the inactive chemicals occupy space in the chips which might otherwise be occupied by the active ones.- In order to introduce into the chips suiiicient active chemicals4 to effect a `pulping action with the ordinary ln'aft cooking liquor, it is hence necessary to use a more c oncentrated liquor than is necessary when the pores `proceeds because of osmosis, as the raw wood chips are wet and the tendency is for chemicals in the cooking liquor to diffuse by osmotic pressure into the pores and interstices of the chips. The point is that when the cooking liquor contains only active chemicals, osmosis forces only active chemicals into the chips, whereas, when inactive chemicals as well as active ones are present in the cooking liquor, osmosis forces both kndsof chemicals into the chips so that it becomes necessary to provide a suiiciently higher concentration of active chemicals to increase the osmotic pressure tending to force active vchemicals into the chips and thereby to arrive at an active chemical content in the chips adequate for the desired pulping action. Assuming that cooking liquors are prepared in accordance with my invention to contain las much active chemicals as the ordinary kraft cooking liquors, it is possible `to eiTect a pulping action under given conditions of temperature and pressure in a shorter period of time than heretofore, wherefore, my invention makes possible an increased pulp reduction in a plant of. a given digester capacity.

The principles of the present invention are also of great value when the production of kraft pulp is being carried on simultaneously with pulp-refining operations wherein alkaline liquors of the same general composition as kraft cooking liquors are used, asillustrated in the iow sheet designated as Figure 2 of the accompanying drawings. In reningrpreliberated wood pulp or the like with alkaline liquors, it is the practice to suspend the pulp in the alkaline liquors under time, temperature, and concentration of liquor conditions designed to effect the desired reaction upon and solution'of the less resistant celluloses, including beta and gamma cellulose, lignin, resins, etc., from the pulp. As a result of such refining action, spent liquors are produced containing dissolved therein .a comparatively small 2,072,177 amount 'of organic material. When such pulpof its alkali content being active, it can be used to good advantage as part of the kraft cooking liquor. The spent refining liquor is quite dilute by reason of the fact that the strong alkaline liquor of a composition similar to that used in kraft cooking is diluted when it is admixed with the aqueous suspension of the .pulp to beV refined thereby and is again diluted when the refined pulp is separated by washing yfrom the spent refining liquor. The spent refining liquor contains sodium sulphate and sodium carbonate, because an alkaline liquor similarv tothe usual kraft cooking liquor has been used in the refining operation. It has been attempted to use the dilute spent refining liquor as part of the' aqueous medium for dissolving the smelted compounds to be causticized, but this has occasioned trouble on account of the fact that the dissolved organic matter carried by the spent rening liquor inhibits to a great extent the settling of the calcium carbonate precipitated in the course of causticization. In other words, the dissolved organic matter coming from the Yspent refining liquor serves as a dispersing or stabilizing agent for the precipitated calcium carbonate, tending to keep it permanently in colloidal suspension; Accordingly, as already indicated, the best prac tice heretofore known was to circulate the spent 'refining liquor to the digesters so as to serve as one of the components of the fresh cooking liquor preparedftherein. Because the spent rening 40 liquor tendsto throw out organic matter as its active chemical content is consumed during kraft cooking, the evaporating systembecomes fouled by the deposition therein of such coagulated organic material as a fine powder; and'the efllciency of the system is impaired as the organic matter accumulates therein along with crystallized solids more particularly in the last-stage evaporators, tending to coat the crystals and to keep thewash water used to clean the system periodically fromgetting at and dissolving the crystals.

I have found that it ispossibleto rid the dilute spent refining liquor substantially completely of its organic matter by adding thereto the ground kiln-reaction products as a concentrated slurry containing barium sulphide and/or barium hydroxide in amount greatly in excess of that theoretically necessary to convert the inactivev chemicals present in the refining liquor, namely, the sodium sulphate and the sodium carbonate, into caustic soda and/or sodium lwdrosulphide. Under these` conditions, the dissolved organic matter is coagulated and the precipitated barium sulphate and barium carbonate entrains and carries down therewith the coagulated organic matter. The resultingscausticized solution containing the residual large amount of barium sulphide and/.or barium hydroxide may then be advantageously added to a sufficient amount of solution of the smelted compounds to react upon and consume all the residual barium sulphide and/or barium hydroxide inthe causticization4 of the? .sodium carbonate and sodium sulphate 4present in the solution of smelted compounds, thereby V-producing a causticized solution `containing no barium constituent whatever. barium sulphate and barium carbonate associated with organic matter is separated from the causticized refining liquor and is subjected to reducing action in a kiln, as hereinbefore described, to regenerate barium sulphide and/or barium oxide, the organic matter thus being consumed in the kiln rather than flowing through the system and functioning as a reducing agent in the reducing reaction effected in the kiln. The essential feature in such recovery procedure is the use of a large excess of the barium sulphide and/or barium hydroxide in concentrated condition in treating the dilute spent refining liquor, for I have found that when such refining liquor is added to the solution of smelted compounds` and a treatment of the mixture is effected with lbarium sulphide and/or barium hydroxide, no

coagulation oi organic matter occurs and, moreover, there is a tendency to inhibit to a large extent the settling of the precipitated barium sulphate and barium carbonate. In other words. the barium sulphide and/q1` barium hydroxide function to coagulate the organic material only when they are present beyond a particular concentration, which particular concentration does not .prevail when the refining liquor is admixed i with a large volumeof liquor containing the smelted compound before the addition of the barium sulphide and/or barium hydroxide. The i causticized solution originating from a mixture' of the refining liquor land the solution of smelted compounds and produced accordant with my invention may be used in preparing fresh refining liquor and fresh kraft cooking liquor, whereas the sediment of barium sulphate and barium carbonate is subjected t'o reducing action in a kiln, as previously described.

Once the principles of my invention have been put into practice in an already-operating kraft mill or in a kraft mill interrelated with a pulprefining mill, it is possible to rid the system The sludge of y substantially completely of inert chemicals, namely, the sodium sulphate and/or sodium carbonate' occurring in the liquor throughout the system. Of course, ythis will take place only after the liquor has gone through the complete cycle a number of times under the conditions of the present invention hereinbefore outlined. As the `inert chemicals are eliminated from the system, the varioustroubles hereinbefore mentioned as being incident to the presence of such chemicals are ,i done away with and such dimculties as arise from forcing the furnace to produce sodium sulphide are also avoided.

An important feature of the present invention is that the heated gases emanating from the kiln in which the barium sulphate and/ or barium carbonate are being reducedv can be advantageously utilized in decomposing the calcium carbonate resulting from the initial stage of causticization into lime. In this connection, I wish to point out that the reduction of barium sulphate and/or f barium carbonate can be edected only at ex,-

tremely high temperatures, whereas calcium carbonate can be decomposed into lime at much lower and are of sumci'ent volume to effect a decompo`. .sition of substantially all the calcium carbonate put into the calcium kiln. f

I have illustrated diagrammatlcally and conventionally in Figure 3 ofzthe accompanying drawings two interconnected kilns running in tandem, one kiln being for the barium and the other being for the calcium y In Figure 3 of the accompanying drawings, the numeral I represents the barium kiln which may be of the.; usual rotary type whose discharge end is journalled for rotation in a combined header and hopper II vmounted on wheels I2 so as to be quickly removable from the discharge end of the kiln. The numeral I3 represents the calcium kiln, which may also be of the usual rotary type, whose discharge end is journalled for rotation in a sleeve or collar I4 having a hopper outlet I5. The intake end of the calcium kiln is journalled for rotation in the collar I4, being provided with l5 an inwardly projecting annular ange I6 and being sufciently spaced from the discharge end of the calcium kiln I3 to avoid commingling of the solid constituents of the two kilns. The intake end of the calcium kiln is journalled forro- 'tation in a gas-receiver Il, being provided also with an inwardly projecting annular ilange I8 for keeping the solid constituents in the kiln. Calcium constituent may be fed into the kiln I3 through a sluice I9 passing through the receiver I'I and discharging on the bottom of the kiln adjacent to the flange I8. Barium constituent may be fed into the kiln Ill through a sluice 20 passing through the upper wall of the collar I4 and discharging on the bottom of the kiln near the annular ilange I6. Combustible gas, preferably producer gas, is delivered into the discharge end of the kiln I0 from a pipe 2| entering into the header II; may be introduced through a pipe 22 entering the side wall of the header. The' pipes 2| and 22 may terminate at the same levels, as shown, whereat they may make a tight sliding fit with the corresponding supply pipes (not shown) so as to permit removal of thevheader from the discharge end of the kiln Ill, as previously described; The removability of the header II from the discharge end of the kiln I0 is desirable for the reason that it makes possible the relining of both kilns and easy access to the interior of the barium kiln wherein sintering or partial clinkering of the solids may occur under the high temperatures prevailing therein so as to necessitate cleaning-of the kiln or` the removal of sintered obstructions therefrom. 'I'he gaseous products emanating from` theintake end of the barium kiln, which pass through the calcium kiln, may require additional air for complete combustion so that a secondary air pipe 23 is shown entering into the upper wall of the collar I4 to deliver the volume of air req' uisite for this purpose. Both the hopper I5 and the hopper portion of the header I I are preferably oiset to one side so as to discharge the solid re- .action products delivered thereinto into suitable tanks stationed'to one side and on a level below .60 the kilns. The hopper I5 discharges into 'a comparatively large hopper-bottomed agitator tank 24 equipped with an'inlet pipe 25 at its top, a

-power driven agitator 26, and a discharge pipe 21 at its bottom. The hopper portion of the header 'I'he air necessary for burning the gasI 'A possible, whereupon they may be used advantageously in drying the sludges going to the calcium and barium kilns, as their 'temperature is still suiilciently high for this purpose.

As previously stated, the initial stageof causticization of the solution of smelted compounds is productive of a sludge consisting essentially of calcium carbonate, but it is contaminated by small amounts of inert lime, sand, calcium silicate, etc. In order to avoid accumulation of impurities in the lime producible from the sludge, the sludge v by a Dorr thickener," whereupon the thickened suspension may then be further reduced in its water content by passage through a lter, preferably a rotary filter of the Oliver type. 'I'he puried and thickened sludge of calcium carbonate, preferably after drying, as hereinbefore described, or after drying by any other hot ue gas, may then be passed through the sluice I9 into the kiln I3 for decomposition into lime. The sludge of barium carbonate,barium sulphate, and carbon resulting from the second stage of causticization may be thickened as on a rotary filter of the Oliver type, sumcient finely divided carbon or carbonaceous material added thereto to ensure the desired reduction to barium oxide and barium sulphide, and the mixture, preferably after drying as hereinbefore described, delivered through the sluice 20 into the kiln I0. The producer gas introduced into the kiln I0 for reducing 'the' barium carbonate .and vbarium sulphate to barium oxide and barium sulphide is burned,

with air to producela maximum temperature in the kiln compatible with minimum sinterlng or clinkering of the solid reaction products, for instance, a temperature approaching about 2500 F. Theaproducer gas is fed into the barium kiln in a volume 'sufficient to ensure the desired reduction in such kiln, .whiclr is accompanied by the liberation of carbon monoxide. 'Ihe gases flow ing into the calcium kiln from the barium kiln should be at sumciently high temperature and should have a sumciently large content of carbon monoxide and/or residual producer gas so that `when they are burned in the calcium kiln with the secondary air furnished through the supply pipe 23, they will create a sufllciently high temperature and liberate sumcient heat to cause a substantially complete decomposition of the calcium' carbonate in the calcium If suillcient air were added along with the producer gas in the barium kiln to consummate a vsubstantially l complete combustion of the producer gas and the carbon monoxide liberated in the bariumskiln, an oxidizing atmosphere would exist 'at the discharge end of the barium kiln so as to preclude the desired reducing action' thereat; a/nd this difficulty would be aggravated bythe lower 'temperature created thereat on account of the dilution with unnecessary comparatively cool air `at this point. It fs'thus seen that the feature of adding secondary-'air to the incompletely burned lgases entering" into the calcium kiln is important from two lstandpointa-namely, that of ensuring the desiredreducing action'and temperature at ,the discharge end of the barium'kiln and that oi'v maintaining sumcient fuel value fand/or a sufliciently high temperature in the gases ilowing vinto the calcium kiln. It is further seen that the barium kiln is maintained under the reducing atmosphere necessary therein, whereas the calcium kiln is maintained under an oxidizing atmosphere such as ensures a complete combustion of the gases and a temperature conducive to the decomposition of the calcium carbonate into lime.

to convert the line into a thick slurry capable of being pumped to storage tanks from which it may be delivered in the desired amount to a Y causticizing tank to which the solution of smelted compounds is sent. In either case, the commingling of the hot lime with Water is accompanied by the liberation of steam which rises through the hopper I5 into the kiln I3, thereby preventing carbon dioxide from reaching the tank -24 and carbonating the lime.

The mixture of barium oxide, barium sulphide and residual barium carbonate, barium sulphate, carbon, and barium silicate, constituting the solid-reaction products of the kiln I0, are delivered into the` comparatively small tank 28 which is supplied with an amount of `Water or preferably an amount of weak barium sulphide solution, presently to be described, lto form 'a thick slurry or suspension which can be handled by a centrifugal pump and piped where desired. As in the tank 24,l there is an evolution of steam inthe tank 28 as the hot reaction products commingle with the water suicient to rise into the header II and thereby to prevent carbon dioxide from gaining access to the tank 28 and carbonating the barium oxide or barium sulphide delivered thereinto." Should the steam liberated in either the tank 24 or the tank 28 be insufficient to preclude the entrance of carbon dioxide thereinto, additional outside steam may be supplied thereinto for this purpose, as througha steam pipe 24a entering throughthe top of the tank 24 and as through a steam pipe 28a entering through the top of` the tank "28. The thick slurry or suspension may be delivered frpm the tank 28. to a ball mill, roller mill, or other pulverizing machinewso as to reduce such i clinkers'as may be present in the slurry to ne .particles whose soluble content, including barium oxide and barium carbonate may be leached out by water. To this en'd, the slurry of ne particles discharged from the pulverizing machine may be used directly or be deliveredto a dissolving tank whereto Water in amount sufficient to ensure the solution of substantially all the soluble compounds present in the slurry may be eected. The solution thus produced containing the insoluble residual barium carbonate, barium sulphate, carbon, and barium silicate in' suspension therein, is separated from the suspended solids and the slurry of solids is then run over a rotary filter of the Oliver type. The thickened slurry is washed on the same lter and the Washings are preferably used, asheethe sluice 20 into the barium kiln. The solution of barium hydroxide and barium sulphide or the pulverized slurry of reaction products from the barium kiln may be used for effecting the second stage of causticization, for vcoagulating organic matter from spent refining liquors, or for causticizing the make-up sodium sulphate, as has hereinbefore been described in detail. It will be appreciated that the tank 28 .serves various functions, including that of permitting the white hot reaction products from the kiln l0 to be quenched therein, .that of permitting the barium oxide to be hydrated therein, that of preventing carbonation of the barium oxide and barium sulphide, and that of a receiver in which the reaction products are converted to a form easily deliverable to other parts of the system.

While I have dealt with the application of the principles of the present invention to the production of kraft pulp and the regeneration of the spent chemical liquors arising therefrom either independently or correlated with the production of refined cellulose pulps and the regeneration of the spent chemical liquors resulting from the refining operation, nevertheless it is to be understood that they are applicable to the vproduction of soda pulp ,and to the regeneration of the spent liquors arising therefrom, as in such case, too, one may advantageously resort `to "a twostage caustic'ization, the first stage of which involves the use of lime and is incomplete and the second stage of which involves the use of barium hydroxide and goes to completion. Indeed, my invention may be extended to various processes of chemical pulp production and the regeneration of the, spent chemical liquors resulting therefrom, lso long as caustic soda is one of the ingredients of the `cooking liquor and causticization 'must be performed `on sodium carbonate produced either by smelting the inorganic content of the spent cooking liquors or by reducing the spent cooking liquors to black ash which is leached with Water for the extraction of its sodium carbonate content. Indeed, my invention may be applied to any process wherein solution of sodium carbonate and/or so dium sulphate is to be causticized to completion,

1. A process which comprises first causticizing a solution of -sodium carbonate and sodium sulphate with calcium hydroxide, and then treating the solution with bariuml hydrosulphide to convert residual carbonate and sulphate into hydrosulphide.

2. A process which comprises first causticizing a solution of sodium carbonate with calcium hydroxide, and then treating the solution with barium hydrosulphide to convert residual car bonate into hydrosulphide.

3. A process which comprises treatinga solution containing dissolved organic matter with barium hydrosulphide, thereby coagulating the dissolved organic matter, said organic matter being that dissolved from cellulosic material and being present in suflcient concentration to be susceptible of the aforesaidl coagulation.

4. A process which comprises treating a solution containing sodium carbonate and sodium sulphate wherein dissolved organic matter is present with barium hydrosulphide, thereby coagulating the dissolved organic matter and converting said sodium carbonate and sodium sulphate to sodium hydrosulphide, said organic matter being that dissolved from cellulosic ma-1 terial and beingbresent in suicien't concentra` vtion to be susceptible of the aforesaid coagulation.

5. A process which comprises treating asolution containing sodium carbonate wherein dissolved organic matter is present with barium hydrosulphide, thereby coagulating the dissolved organic matter and converting said sodium carbonate to sodium hydrosulphide, said organic matter being that dissolved from cellulosic material and being present insufficient concentration to be 'susceptible of the aforesaid coagulation.

6. A process which comprises treating a solution containing sodium sulphate wherein dissolved organic matter is present with barium hydrosulphide, thereby coagulating the dissolved organic matter and converting said sodium sulphate to sodium hydrosulphide, said organic matter being that dissolved from cellulosic material and being present in suiiicient concentration to be susceptible of the aforesaid coagulation.

7. A process which comprises smelting the inorganic content of spent `krait cooking liquor to produce a smelt containing sodium carbonate and sodium sulphate, dissolving the smelt in water, causticizing the resulting solution with calcium hydroxide, and then treating the solution with barium hydrosulphide to convert residual carbonate and sulphate into hydrosulphide.

8. A process which comprises smelting the inorganic content of spent sodium base cooking liquors to produce a smelt containing sodium carbonate, dissolving the smelt in water, 'causticizing the resulting solution withl calcium hydroxide, and then treating the solution with barium hydrosulphide to convert residual carbonate into hydrosulphide.

9. A process which comprises smelting the inorganic content of spent kraft cooking4 liquors to produce a smelt containing sodium carbonate and sodium sulphate, dissolving the smelt in water, causticizing the resulting solution with calcium hydroxide, and then reacting upon residual sodium carbonate and sodium sulphate in the resulting solution atv least in part with barium hydrosulphide. l

. 10. A process which comprises 'smelting the inorganic content of spent sodium'base c'ooking liquor to produce a smelt containing sodium carbonate, dissolving the smelt in water, causticizing the resulting solution with calcium hydroxide, and then reacting upon residual sodium carbonate in the resulting solution at least in part with barium hydrosulphide.

11. A process which comprises smelting the inorganic content of spent kraft cooking liquor to produce a smelt containing sodium carbonate and sodium sulphate, dissolving the smelt in water, causticizing the resulting solution substantially as far as possible with calcium hydroxide, and then converting residual sodium carbonate and sodium sulphatein the resulting solution to sodium hydrosulphide by the'addition of barium hydrosulphide thereto.

12. A process which comprises smelting the inorganic content of spent sodium base cooking liquor to produce a smelt containing' sodium carwhich comprisesmelting the inorganic content of the spent pulping liquor to form a smelt containing sodium carbonate, dissolving the smelt in Water, causticizing the resulting solution with lime, treating the spent pulp-renng liquor A with an excess of barium h'ydroxide and barium hydrosulphide to' coagulate the organic matter present therein, and commingling the treated spent pulp-refining liquor with the lime-causticized solution to consume substantially all of the residual barium hydroxide and barium hydrosulphide in converting residual sodium carbonate present in the lime-causticized solution to caustic soda and sodium hydrosulphide, thereby reproducing a liquor containing caustic Isoda and sodium hydrosulphide for vboth the pulping and pulp-rening operations. y

14. In the pulping of raw cellulosic material with liquor containing caustic soda and sodium hydrosulphide. those steps which comprise smelting the inorganic content of the spent pulping liquor to produce a smelt containing sodium carbonate, dissolving the smelt in water, causticizing the resulting solution substantially as far as possible with lime, and treating lthe limecausticiz/ed solution with barium hydroxide and barium hydrosulphide to convert residual sodium carbonate present in the solution to caustic soda and sodium hydrosulphide, thereby reproducing a liquor for the pulping operation.

15. In the pulping of raw cellulosic material with liquor containing caustic soda and sodium hydrosulphide, those steps which comprise smelting the inorganic content of the spent pulping liquor to produce a smelt containing sodium carbonate, dissolving the smelt in water, causticizing thelresulting solution substantially as far as possible with lime, adding sodium sulphate thereto,-

and treating the solution with barium hydroxide and barium hydrosulphide to convert the sodium .sulphate and residual sodium carbonate present in the solution to caustic soda and sodium hydrosulphide, thereby reproducing a liquor for the pulping operation. l

16. In a cyclic process, steps which comprise pulping raw cellulosic material in a solution containing caustic soda and sodium hydrosulphide, concentrating by evaporation the resulting spent pulping liquor, smelting the inorganic content of the 'concentrated liquor, dissolving the smelted compounds in water, causticizing the resulting solutionwith lime, and, by the action of barium hydroxide and barium hydrosulphide, converting residual uncausticized compounds present in the lime-causticized solution and tending to interfere with pulping, to crystallize out during the step of concentration, and to cause trouble during the step of smelting, into caustic soda and sodium hydrosulphide having far less such tendency before repeating the cycle.

17. In a cyclic process, steps which comprise pulping raw cellulosic material in a solution containing vcaustic soda and sodium hydrosulphide, concentrating by evaporation the resulting spent pulping liquor, smelting the inorganic content of the concentrated liquor, dissolving the smelted compounds in water, and, by the action of both calcium hydroxide and barium hydrosulphide,

`converting practically all of such compounds into caustic soda.and sodium hydrosulphide before repeating the cycle, thereby avoiding troubles in the pulping, concentrating, and smelting steps arising from the presence of residual uncausti-.N'

clizecl compounds in said solution of smelted com-v pounds when said solution is causticized only with calcium hydroxide. i l

` 18; In a cyclic process, steps which comprise pulping raw cellulosic material in a solution containing caustic soda and sodium hydrosulphide, concentrating by evaporation the resulting spent pulping liquor, smeltng the inorganic content of the concentrated liquor, dissolving the smelted compounds in'water, causticizing the resulting 

